Like the game tip says pinning them down with a melee and artillery tower works well.

In extreme difficulty and events, it's best to build a level 3 artillery when possible because level 2 will barley dent them.

In event maps where scrap is low starting off with a level 2 melee tower can hold them in place for your champions to nuke them with his/her abilities. However this leaves you vulnerable to Speeder Bikes and mass amounts of troops, so carry Orbital Bombardments if they overwhelm you.